PmWiki requires every page to be a member of a group. A group is like a wiki within a wiki; it can have its own presentation look, security controls and navigation aids. With default configuration, WikiWords are only searched inside the current group, and you use either OtherGroup/MyWikiWord or OtherGroup.MyWikiWord to refer to pages in other groups (see Links).

Individual wiki pages can also be grouped by having tags and links to a common "category" page; we say that any pages that link to a common page are in a "category" defined by that page. PmWiki uses the [[!category]] markup as a shorthand to place a page into a category with other pages containing the same markup.

The shortcoming of categories is that categories do not distinguish between the declaration of a category ([[!structure]]) and the link to a category ([[Category/Structure]]).